Field of the Invention
The present invention relates to metal nanotubes and metal nanowires useful as fuel cell catalysts, particularly in proton or hydroxide exchange membrane fuel cells, and fuel cells comprising the catalysts.
Description of the Related Art
Optimizing energy demand within the transportation field is a significant worldwide concern and especially crucial within the United States (US). The transportation sector globally accounts for 20% of energy demand and relies heavily on fossil fuels.[1] In 2008, oil prices reached historic levels in part due to a strong increase in demand for transportation fuels.[2] Transportation is of particular concern in the US as the US accounts for 28% of worldwide transportation energy consumption.[3] Within US energy, 29% (27.03 quadrillion BTUs) is consumed in transportation and 83% is supplied by fossil fuels.[3, 4] Though the US is the primary user of fossil-fuel based transportation, from a global perspective, continual increases in worldwide demand will strain fuel cost. It is anticipated that developing nations, or countries identified within the non-Organization for Economic Cooperation and Development group, will significantly contribute with an annual inflation rate of 2.6% in transportation energy demands (2007-2035).[1]
Although a timeline for global peak oil production is not universally accepted, it is generally understood that peak oil production among countries outside of the Organization of Petroleum Exporting Countries has already passed, compromising energy security and impacting fuel costs.[5] For example, the US currently produces (5.3 million barrels per day in 2009) 55% of its peak (1970) production, but the US petroleum demand continued to increase by 19% over the same time frame.[3] In an effort to lessen the burden of fossil fuels on the transportation field, fuel cells have been examined as a potential technology to develop non-fossil fuel based transportation devices.
Fuel Cell Structure and Electrolyte Membranes
FIG. 57 illustrates a typical fuel cell with an anode portion (illustrated on the left) and a cathode portion (illustrated here on the right) which are separated by an electrolyte; supporting members are not illustrated. The anode portion carries out an anode half-reaction which oxidizes fuel releasing electrons to an external circuit and producing oxidized products; the cathode portion carries out a cathode half-reaction which reduces an oxidizer consuming electrons from the external circuit. The gas diffusion layers (GDL) serve to deliver the fuel and oxidizer uniformly across the catalyst layer. Charge neutrality is maintained by a flow of ions from the anode to the cathode for positive ions and from cathode to anode for negative ions. The dimension illustrated is for convenience and is not representative, as the electrolyte membrane is usually selected to be as thin as possible consistent with membrane structural integrity.
In the case of the illustrated hydroxide exchange membrane fuel cell (HEMFC), the anode half-reaction consumes fuel and OH− ions and produces waste H2O (also CO2 in the case of carbon containing fuels); the cathode half reaction consumes O2 and produces OH− ions; and OH− ions flow from the cathode to the anode through the electrolyte membrane. Fuels are limited only by the oxidizing ability of the anode catalyst but typically can include H2, MeOH, EtOH, ethylene glycol, glycerol, and similar compounds. Catalysts are usually Pt or based on Ag or one or more transition metals, e.g., Ni. In the case of a PEMFC, the anode half-reaction consumes fuel and produces H′ ions and electrons; the cathode half reaction consumes O2, H+ ions, and electrons and produces waste H2O; and H+ ions (protons) flow from the anode to the cathode through the electrolyte membrane. For such fuel cells, fuels are most commonly H2 and MeOH.
PEMFCs have been identified as a transformative technology intended to create a new paradigm in the way energy is used. Although not limited to the transportation field, PEMFCs are well suited as transportation devices due to a low operation temperature and high energy density. Commercialization of this technology is principally limited by high material costs and low catalyst durability.[6, 7] These obstacles hinder the technology's application in the transportation field as they pose significant technological and financial risks to industry. In order overcome such developmental obstacles and to advance PEMFC commercialization, a technological breakthrough is required.
Utilizing hydrogen has allowed for such a breakthrough as it has resulted in the diversification of energy sources and production methodology; methods of hydrogen production include, but are not limited to fossil fuel reformation, the Kværner-process, electrolysis, solar, nuclear, and biological. As a beneficial PEMFC fuel source, hydrogen has an extraordinarily high specific energy density (Table 1) resulting in fuel cell vehicles that can provide the same output as conventional combustion engine vehicles while using half the energy input.[8-10] Estimations of the Government Performance and Results Act conclude that oil savings of 5.3 million barrels per day can be achieved from the use of light duty fuel cell transportation vehicles by 2050, assuming a 37% market penetration.[11]
TABLE 1Specific energy densities of selected fuel sources.WeWe[MJ kg−1][MJ L−1]Hydrogen, g142.00.013Methanol21.917.3Ethanol28.822.7Ethylene glycol19.021.2Gasoline, auto45.833.9Natural gas46.60.037Coal, anthracite31.435.0
Since PEMFCs combine hydrogen and oxygen to produce the power output, water is the only emission. Previous studies have established that carbon dioxide generation can be significantly reduced using fossil fuels for hydrogen production in fuel cells as opposed to transportation utilizing fossil fuels directly.[12] Greenhouse gas emissions from hydrogen production are generated at a single point instead of at each end use application. Thus, on the occasion that fossil fuels are used for hydrogen production, the formation of greenhouse gases can be properly treated or sequestered at the singular production site. Overall, utilizing PEMFC based transportation would result in an end use application that is a zero emission entity—entirely eliminating any greenhouse gas emission contributions. An analysis conducted on a well-to-wheels basis asserted that fuel cell vehicles produced from fossil fuels produce 42% and 60% less carbon than hybrid electric and conventional internal combustion engine vehicles, respectively.[13]
In addition to PEMFCs, alternative fuel cell configurations traditionally include, but are not limited to solid oxide, alkaline, and direct alcohol fuel cells. Among these options, PEMFCs are the most suitable for the transportation field: solid oxide fuel cells require high temperatures for efficient operation; liquid alkaline electrolyte use requires a closed environment; and alcohols have a specific energy density less than hydrogen and traditional transportation fuels (Table 1).
PEMFCs utilize Pt for ORR and the hydrogen oxidation reaction (HOR). For PEMFC development, highly active cathode catalysts are of particular interest since the overpotential for ORR is significantly larger than HOR; it has previously been suggested that the stability of adsorbed oxygen at high potentials prevents proton and electron transfer and creates the observed ORR overpotential.[14, 15] Pt is regarded as the most active ORR catalyst, in part due to a nearly optimal binding energy with oxygen and hydroxide.[16] Although non Pt and non noble metal catalysts have been examined, larger overpotentials are generally observed, particularly in an acidic electrolyte.[17, 18] Early Pt development, therefore, focused on the reduction of particle size to 2-3 nm, thereby increasing surface area and ORR mass activity. The reduction in particle size, however, decreased the ORR specific activity and the improvements in mass activity were disproportionate to the surface area.[19, 20] Therefore, further particle size reduction cannot ensure the commercial viability of PEMFCs. In order to promote the development of Pt catalysts with high ORR activity, the DOE set benchmarks (2010-2015) for mass (0.44 Amg−1) and specific (0.72 mAcm−2) activity.
In addition to cost concerns, PEMFC commercialization is also limited by catalyst durability. The loss of ORR activity and surface area in PEMFC cathodes has been studied previously; Pt/C degradation can be categorized into the following areas: erosion of the carbon support; surface tension driven nanoparticle agglomeration; Ostwald ripening; and potential driven Pt dissolution and migration into the membrane.[21]
Recent developments of Pt nanomaterials have led in two promising directions: extended Pt networks that can improve ORR activity and durability; and Pt films that can decrease the catalyst cost.[22-26] Sun and Wang et al. recently synthesized Pt tetrahexahedrons, tailored from 20 nm to 240 nm in diameter by the electrochemical treatment of nanospheres; although the tetrahexahedrons were not studied for catalytic activity, the synthesis allowed for controlled facet growth.[22] Xia et al. studied Pt Pd nanodendrites, approximately 20 nm in diameter; the ORR activity, however, does not meet the DOE benchmarks and catalyst durability suffered due to the presence of Pd.[23] In Pt coatings, Adzic and Mavrikakis et al. electrochemically applied Pt monolayers to films of ruthenium, iridium, rhodium, gold, and Pd, finding improved ORR activity in the case of Pd.[24] Adzic et al. further applied a Pt layer to cobalt (Co) Pd core shell nanoparticles, thereby improving ORR activity.[25] Nørskov et al. also examined polycrystalline Pt films alloyed with Co, nickel, iron, vanadium, and titanium.[26] While the specific ORR activity of the Pt3Co film was three times greater than pure Pt, each of the preceding publications required electrochemical synthesis and is impractical in an industrial setting. Additionally, fundamental studies were previously conducted on Pt facet activity and lattice tuning. Marković et al. found that ORR activity on low index Pt surfaces increased in the order {100}<{111}<{110}.[27] Mukerjee et al. also modified Pt—Pt bond distances and d-orbital vacancies with the introduction of metal alloys; it was determined that lattice and electronic tuning impacted ORR activity.[28]